Noise limiter circuit



- FB H, 1941. E. l. ANDERSON 2231,37?

NOISE LIMITER CIRCUIT Filed April 7, 1938 AVC AAAAA AA TOAMPLGR/DS I 70 4.5 NETWORK PLATE MILL/AMFERES PLATE VOLTS I NV EN TOR.

EAR?NDER$0N BY WW' ATTORNEY,

Patented Feb. 11, 194-1 UNITED STATES NOISE LIMITER CIRCUIT ware Application April 7, 1938, Serial No. 200,616

6 Claims.

My present invention relates to wave amplitude limiter circuits, and more particularly to a radio receiver employing a demodulator adapted to act as a noise limiter.

One of the main objects of my present invention is to provide a wave amplitude limitingdevice in a wave transmission system, the device comprising an electron discharge tube provided with a cathode, an output electrode and additional electrodes for imparting to the tube a limiting characteristic; the waves to be transmitted through the system being impressed between the cathode and output electrode of the tube.

Another important object of my invention is to provide a wave transmission system wherein advantage is taken of the fact that the plate resistance of a pentode tube is very high for plate voltages in excess of a certain critical voltage and very low for lower voltages; and the pentode tube being employed in the system at a point where wave amplitude limiting action is desired.

Another object of my invention may be stated to reside in the provision of a radio receiver equipped with a detector comprising a pentode tube; the radio frequency waves being applied between the plate and cathode of the pentode tube to provide a demodulated output; and the current through the tube being proportional to plate voltage for values of voltages less than a predetermined critical value, but increasing only very slowly for voltages higher than a critical value.

Still another object of my invention is to provide a noise limiter circuit in a superheterodyne receiver; the second detector employing a pentode tube between whose cathode and plate is provided a diode detection network; and the screen electrode voltage of the pentode tube being variable to adjust the amplitude limiting point of the detector in accordance with the signal level and noise conditions.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my inventionmay be carried into effect.

In the drawing:

Fig. 1 shows the second detector network of a ,superheterodyne receiver embodying the invention,

Fig. 2 illustrates a family of plate voltage-plate current characteristic curves of the second detector tube.

Referring specifically to Fig. 1 there is shown in the latter the second detector network of a su- 5 perheterodyne receiver; and for the sake of simplicity of disclosure and description, the remaining networks of the receiver are not shown. Of course, a tuned radio frequency amplifier type of receiver may be used. Those skilled in the art 10 are fully aware of the nature of the omitted networks, and it need only be stated that any well known. circuits usually employed in superheterodyne receivers may be utilized prior to, and following, the second detector network. The second 15 detector tube itself is a pentode tube and is designated by the numeral l this tube may be of the 6J7 type. The plate 2 of the pentode tube is connected to the high alternating potential side of the input circuit 3. The low potential end of input circuit 3 is connected to the grounded cathode of tube I through a path which includes the intermediate frequency (I. F.) choke coil 5 and the load resistor '5. An I. F. bypass condenser 6 is connected to ground from the low potential end of input circuit 3.

The input circuit 3 is resonatedto the operating I. F., and the latter may have any frequency between '75 and 450 K. C. The numeral 7 designates the resonant output circuit of a preceding I. F. amplifier, and circuit 1 is tuned to the same I. F. as is circuit 3. It will be understood that the circuit 1 may be preceded by a transmission system which may comprise a signal collector circuit; one or more stages of tunable radio frequency amplification; a tunable first detector which is fed with local oscillations from a tunable local oscillator; and one, or more, amplifier stages tuned to the operating I. F. value.

The detected current flowing through load resistor 5 develops a voltage thereacross; the audio voltage component is transmitted by the adjustable audio tapping device 8 to one, or more, stages of audio frequency amplification. The latter may be followed by any desired type of reproducer, such as a loudspeaker. The direct current voltage component of the voltage developed across load resistor 5 is employed for automatic volume control action (AVC), and for this reason the lead 9 is to be understood as being connected 50 from a point on resistor 5, which is negative with respect to ground when signals are impressed on circuit 3, to the signal grids of any of the tubes preceding circuit 3 whose gain is to be under control. The numeral [0 denotes'the usual time constant network and pulsating voltage filter which is customarily employed in an AVC connection. If desired, the AVG connection may be made to the signal grids of the radio frequency amplifier, first detector and I. F. amplifiers. As is well known to those skilled in the art, the AVG connection functions to vary the gain of the controlled stages in such a manner that the carrier amplitude at the input circuit 3 is substantially uniform over a wide range of signal carrier amplitude variation at the signal collector.

The tube I, being of the pentode type, includes a grid ll which functions as a suppressor grid, and is at cathode potential. The grid I2, positioned between suppressor grid and the first grid l3, functions as a positive screen grid. The grid I2 is shown connected to a source of positive potential; the latter may be a voltage bleeder resistor 54 connected from a point of positive potential on the usual direct current voltage supply network. The connection from grid l2 to bleeder resistor M includes an adjustable tap I5, and the I. F. bypass condenser I6 is connected between the grid I 2 and ground. The grid I3 is connected to ground through a resistor I! which may have a magnitude of the order of 1 megohm. The function of resistor ii is to reduce the effect of contact potential. Adjustment of the tap l5 on bleeder resistor it results in a selection of a particular one of a family of plate voltage-plate current characteristic curves.

In order to explain the manner in which the noise limiting action is secured by the use of the pentode tube l, the family of curves shown in Fig. 2 is first considered. In this figure there is shown a family of plate volts-plate milliamperes characteristic curves for the 6J7 type pentode tube. In a purely qualtitative manner, there is shown the family of curves secured by varying the positive potential on the screen grid l2. t will be noted that the lowermost curve is secured with the minimum screen grid potential, whereas the top curve is produced by using maximum positive potential on the screen grid. From this family of curves it will be clear that on any selected characteristic curve the plate current of the tube will increase proportionately to the increase in plate voltage up to a certain critical value, and subsequent increase in plate voltage will cause substantially no increase in plate current. Such a characteristic prevents noise peaks from being reproduced by the loudspeaker of a receiver, since above a predetermined plate voltage value there will be no further plate current flow.

By adjusting the tap IS on a proper point on bleeder resistor Hi the operator can preselect the wave amplitude above which the voltage developed across resistor 5 will not increase substantially. Advantage is taken of the fact that the plate resistance of a pento-de tube is very high for plate voltages in excess of a certain critical voltage, and very low for lower voltages. This critical voltage value is a function not only of the screen grid voltage, but, also, of the voltage of the control grid I3. If an alternating current voltage be applied to the plate of a pentode tube having a high plate resistance, then the current through the tube will be proportional to voltage for values of plate voltages less than the critical value, but will increase. only very slowly for plate voltages higher than the latter value. By adjusting the screen grid voltage (or the control grid voltage if desired) the critical voltage may be made any desired value.

7 low voltage value.

It will be understood that the screen grid voltage will be adjusted in value depending upon the signal carrier amplitude and the noise conditions. If, for example, weak signals are to be received, and the noise conditions are particularly severe, the tap I5 might well be adjusted to a fairly In this case the current limiting action in tube I would commence at a fairly low plate voltage value. On the other hand if the received signals are strong, and the noise level is not particularly high, then the tap l5 would be adjusted to select one of the uppermost characteristic curves so that current limiting does not begin until the noise peaks which are to be clipped, or limited, exceed a predetermined critical voltage to be applied to the plate 2 of detector tube I.

It is to be clearly understood that the present invention is not limited to the pentode tube specifically. In general, any tube can be employed which possesses a family of plate voltageplate current characteristic curve of the type shown in Fig. 2. In other words, any tube can be employed in the present invention which utilizes diode rectification, and can be adjusted so that its current output is substantially proportional to anode voltage up to a predetermined critical value, and thereafter further increase of anode voltage causes substantially no increase in tube current flow.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a signal receiving system, a detection network comprising a tube having a cathode and anode electrode, a signal input circuit, including a load impedance, connected between the cathode and anode, said two electrodes being at a common direct current potential in the absence of signals, a grid electrode in the electron stream to said anode, means establishing said grid at a constant positive potential, a suppressor grid between the positive grid and anode, and said tube having an anode voltage-anode current characteristic such that the anode current flow is limited for signals above a predetermined amplitude.

2. In a signal receiving system, a detection network comprising a tube having a cathode and anode electrode, a signal input circuit, including a load impedance, connected between the cathode and anode, said two electrodes being at a common direct current potential in the absence of signals, a grid electrode in the electron stream to said anode, means establishing said grid at a constant positive potential, a suppressor grid between the positive grid and anode, and said tube having an anode voltage-anode current characteristic such that the anode current flow is limited for signals above a predetermined amplitude, an auxiliary grid electrode disposed between the cathode and positive grid, and means for establishing the auxiliary grid at cathode potential.

3. In a radio receiver, a carrier amplification network, a detector of the type including a tube provided with a cathode, a plate and a positive cold electrode therebetween, means impressing carrier waves amplified in said network between said cathode and plate for rectification of the carrier waves, a load impedance connecting the plate and cathode and maintaining them at a common direct current potential in the absence of carrier waves, and the constants of said tube and the positive voltage of said cold electrode being at a fixed value so chosen that the plate current of said tube increases with plate voltage up to a predetermined critical plate voltage value and thereafter the plate current remains substantially constant for further plate voltage increase.

4. In a radio receiver as defined in claim 3, additional means for varying the voltage of said cold electrode over a range of positive potential values thereby to provide a family of plate current-plate voltage characteristics for said detector tube.

5. In a demodulation network for a carrier wave transmission system, a tube of the pentode type provided with a cathode, plate and at least three grids therebetween,'means impressing carrier waves between the cathode and plate of said tube thereby to provide a carrier wave rectification circuit, a load impedance connecting the plate and cathode and maintaining them at a common direct current potential in the absence of carrier Waves, means deriving a rectified carrier voltage from said rectification circuit, and means for adjusting the positive direct current potential of at least one of the three grids to a predetermined constant value.

6. In a demodulation network as defined in claim 5, the second of the three grids being a positive screen grid, said adjusting means varying the positive potential of the screen grid, and said tube having a limiting characteristic for carrier waves above a predetermined amplitude value. 

